which of the three stations do you think was closest to the epicenter and why?

Earthquake Hazards in the Eastern U.S.
We often assume, mistakenly as it turns out, that because the eastern U.S. has no active plate boundaries that it has no earthquake risk. While earthquakes in plate interiors are much less frequent, there are some factors that make them even more dangerous than those in more seismically active areas. With this homework exercise we explore an eastern U.S. earthquake and consider some of the factors that can affect societal response to this under-appreciated hazard.
Before You Begin
Go to the Google Earth web site and download Google Earth Pro:
Note: You must install Google Earth Pro to complete the exercise; regular Google Earth does not include the ability to draw circles.
Part I – Locating an Earthquake.
Open the Google .kmz file attached to this Assignment description in Isidore. This should automatically open Google Earth and navigate you to a view of the eastern U.S. This file contains the locations of three seismic stations in the eastern half of the U.S. – SSPA (Standing Stone, Pennsylvania), WCI (Wyandotte Cave, Indiana), and DWPF (Disney World Preserve, Florida). Seismic records are included here from each of these three stations. Note that the S- and P-wave arrival times have been automatically picked using the electronic interpretation algorithms encoded into each of the seismographs and recorded on the right side of each seismogram. Also note that there are actually three seismograms stations at each station – BH1 and BH2 record the lateral motion in perpendicular directions whereas BHZ records the vertical motion. Also note how difficult it would be to pick out the p-wave arrivals by eye (BH! For SSPA is probably the easiest to visually pick out the different phase arrivals).
Study the vertical axis of the various seismograms and record the maximum amplitudes of the highest wave peaks at each of the seismic stations below (Don’t worry about the units – we are only interested in comparing them in a relative sense):
Based on the wave heights, which of the three stations do you think was closest to the epicenter and why?
To locate the epicenter more precisely, we need to figure out how far each station was from the epicenter. To do this, we will use the S-P lag times from each station and the travel- time graph at the end of this hand-out. In the table below, record the P- and S-wave arrival times from the seismograms and use them to calculate the S-P lag times, the distances to the epicenter, and the predicted P- and S-wave travel times from the epicenter, and finally, to estimate the time of the earthquake by averaging the times estimated from each of the three seismic stations.

Station P-arrival S-arrival S-P lag Epicentral Distance P-travel time S-travel time Time of Quake
SSPA 17:51 46 17:52 21
WCI 17:52:38 17:53:54
Est. time earthquake occurred (average time of quake)

Now, with the distances to the epicenters, we have the critical information we need to locate the epicenter. For each of the three seismic stations go into Google Earth Pro, click on the seismic station in question, and then select the “ruler tool,” and select the “Circle” tab (Note: If you do not see the “circle” tab, you are not using Google Earth Pro – you only have this functionality in Google Earth Pro). With the circle measure tool selected, click once on the seismic station in question to set it as the center of the circle and then begin “pulling away” from the seismic station without clicking to enlarge the circle; you will need to continuously “zoom out” at the same time to make the circle large enough. As you enlarge your circle, keep an eye on the Radius of the circle in the toolbox. Once the radium equals your epicentral distance for that seismic station, click again to set the circle and then click “Save” to save it. If you like, right-click the circle and open the “Properties” dialog (PC, of “Get Info” on Mac systems) which will give you an opportunity to give your circle a name, such as “WCI measure circle.” Repeat the same procedures for the other two seismic stations. Now, all three circles should intersect or at least come close to intersecting at one point – do they? Normally there will be a small “error triangle” near the intersection point. Click on the “add place mark” tool (the yellow thumb tack in the menu bar) to locate your best estimate for the earthquake epicenter in the middle of your error triangle.
Record the latitude and longitude of your estimated earthquake epicenter below.

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